Yahoo! UI Library

   /**
    * Extends the IO base class to implement Queue for synchronous
    * transaction processing.
	* @module io
	* @submodule io-queue
	*/

   /**
	* @description Array of transactions queued for processing
	*
	* @property _yQ
	* @private
	* @static
	* @type Object
	*/
	var _q = new Y.Queue(),

   /**
	* @description Reference to "io:complete" event handler.
	*
	* @property _e
	* @private
	* @static
	* @type Object
	*/
	_e,

	_activeId,
   /**
	* @description Property to determine whether the queue is set to
	* 1 (active) or 0 (inactive).  When inactive, transactions
	* will be stored in the queue until the queue is set to active.
	*
	* @property _qState
	* @private
	* @static
	* @type int
	*/
	_qState = 1;

   /**
	* @description Method for requesting a transaction, and queueing the
	* request before it is sent to the resource.
	*
	* @method _queue
	* @private
	* @static
	* @return Object
	*/
	function _queue(uri, c) {
		var o = { uri: uri, id: Y.io._id(), cfg:c };

		_q.add(o);
		if (_qState === 1) {
			_shift();
		}

		Y.log('Object queued.  Transaction id is' + o.id, 'info', 'io');
		return o;
	}

   /**
	* @description Method Process the first transaction from the
	* queue in FIFO order.
	*
	* @method _shift
	* @private
	* @static
	* @return void
	*/
	function _shift() {
		var o = _q.next();

		_activeId = o.id;
		_qState = 0;
		Y.io(o.uri, o.cfg, o.id);
	}

   /**
	* @description Method for promoting a transaction to the top of the queue.
	*
	* @method _unshift
	* @private
	* @static
	* @return void
	*/
	function _unshift(o) {
		_q.promote(o);
	}

	function _next(id) {
		_qState = 1;
		if (_activeId === id && _q.size() > 0) {
			_shift();
		}
	}

   /**
	* @description Method for removing a specific, pending transaction from
	* the queue.
	*
	* @method _remove
	* @private
	* @static
	* @return void
	*/
	function _remove(o) {
		_q.remove(o);
	}

	function _start() {
		_qState = 1;

		if (_q.size() > 0) {
			_shift();
		}
		Y.log('Queue started.', 'info', 'io');
	}

   /**
	* @description Method for setting queue processing to inactive.
	* Transaction requests to YUI.io.queue() will be stored in the queue, but
	* not processed until the queue is reset to "active".
	*
	* @method _stop
	* @private
	* @static
	* @return void
	*/
	function _stop() {
		_qState = 0;
		Y.log('Queue stopped.', 'info', 'io');
	};

   /**
	* @description Method to query the current size of the queue.
	*
	* @method _size
	* @private
	* @static
	* @return int
	*/
	function _size() {
		return _q.size();
	};

	_e = Y.on('io:complete', function(id) { _next(id); }, Y.io);

   /**
	* @description Method to query the current size of the queue, or to
	* set a maximum queue size.  This is the interface for _size().
	*
	* @method size
	* @public
	* @static
	* @param {number} i - Specified maximum size of queue.
    * @return number
	*/
	_queue.size = _size;

   /**
	* @description Method for setting the queue to active. If there are
	* transactions pending in the queue, they will be processed from the
	* queue in FIFO order. This is the interface for _start().
	*
	* @method start
	* @public
	* @static
    * @return void
	*/
	_queue.start = _start;

   /**
	* @description Method for setting queue processing to inactive.
	* Transaction requests to YUI.io.queue() will be stored in the queue, but
	* not processed until the queue is restarted. This is the
	* interface for _stop().
	*
	* @method stop
	* @public
	* @static
    * @return void
	*/
	_queue.stop = _stop;

   /**
	* @description Method for promoting a transaction to the top of the queue.
	* This is the interface for _unshift().
	*
	* @method promote
	* @public
	* @static
	* @param {Object} o - Reference to queued transaction.
    * @return void
	*/
	_queue.promote = _unshift;

   /**
	* @description Method for removing a specific, pending transaction from
	* the queue. This is the interface for _remove().
	*
	* @method remove
	* @public
	* @static
	* @param {Object} o - Reference to queued transaction.
    * @return void
	*/
	_queue.remove = _remove;

    Y.mix(Y.io, {
		queue: _queue
    }, true);